The presence of a 1.4 x 106 dalton double-stranded RNA in intracellular virus-like particles (the K1 killer plasmid or killer virus) in saccharomyces cerevisiae enables cells to secrete a protein toxin and be immune to this toxin. We have defined 29 chromosomal genes (called mak1, . . .) needed to replicate the killer plasmid and 5 regulatory chromosomal genes affecting the killer plasmid (ski genes, KRB1). A second killer plasmid, called K2, is excluded by the K1 plasmid. Another plasmid, called (EXL), is not a killer plasmid but also excludes K2. Yet another plasmid, called (NEX), prevents this effect of (EXL) on K2. Two chromosomal genes, MKT1 and MST2, are required for K2 maintenance if (NEX) is present. A third new yeast plasmid, (HOK), acts as a helper for a replication-defective mutant of the K1 killer plasmid. (NEX) and HOK) are carried on a 3.0 x 106 dalton double-stranded RNA (called LHOK-NEX). Another form of L ds RNA depends on the chromosomal CLO1 gene for its replication. A third form of L ds RNA has also been found.